$^{112}$Sn Double-Electron Capture to Excited States - A Possible Alternative to Neutrinoless Double-Beta Decay

As first suggested by Winter in 1955 and re-emphasized by Georgi, Glashow and Nussinov in 1981 the double-electron capture (EEC) process on a nucleus with an excited state of the daughter at an energy that coincides with the mass difference between the parent and daughter atoms may play a decisive role in determining the properties of neutrinos. For perfect degeneracy, a substantial resonance enhancement in the capture probability is predicted, and there is no phase space left for the two-neutrino double electron capture. Here we concentrate on $^{112}$Sn. Using the TUNL apparatus designed for double-beta decay studies to excited states we placed a 3.9 g foil of enriched (99.5\%) $^{112}$Sn between our two HPGe detectors. In addition, we surrounded the detectors with rods of natural tin (0.97\% of $^{112}$Sn), thus providing a total mass of 15.7 g of $^{112}$Sn for our search for the coincident detection of 1253.4 keV and 617.6 keV $\gamma$ rays (0$^+\rightarrow$2$^+\rightarrow$0$_{gs}^+$ sequence). After 60 days of counting we obtained no events in the energy region of interest. This null result corresponds to T$_{1/2}$ $>$ 1.4 $\times$ 10$^{19}$ years (90\% CL) for the 0$\nu$EEC process. Analysis of the singles spectra, gives a half-life of T$_{1/2}$ $>$ 4.8 $\times$ 10$^{19}$ years (90\% CL) for the 0$\nu$EEC process.